Landslides Examined

The Landslide Blog helps draw attention to landslide events—and the work being done to minimize damage and loss of life.

Jason ZaskyDec 31, 2019

Vajont Dam, northern Italy, 1963. Public domain.

Landslides are a big, broad topic.

“When it comes to landslides we’re talking about everything from the size of a coffee cup to something the size of half a mountain,” begins Dave Petley, Pro-Vice-Chancellor at the University of Sheffield and founder of The Landslide Blog. “We are dealing with a vast array of sizes, but any one of them can cause loss of life.”

Yet landslide events rarely receive the kind of widespread media coverage afforded to hurricanes, earthquakes, tornadoes, floods, blizzards, and other natural disasters. “That’s one of the reasons I started The Landslide Blog. It was an attempt to raise the profile of landslides,” continues Petley.

As such, The Landslide Blog provides a running commentary on landslide events occurring around the world, including discussions of the latest news and research. For example, a post from earlier this year highlighted the fact that July 2019 was a record-breaking month for fatal landslides. That coincided with the warmest July on record, a combination that is sure to prompt further investigation on the part of researchers.

In the meantime, Petley was kind enough to speak with Failure about his experience with landslides and the measures that can be taken to prevent them. Along the way, he addressed the link between changing environmental conditions and an increase in the number of life-threatening landslide events, not to mention what he views as the “most significant” landslide in history, which occurred at Vajont Dam (pictured).

What led you to pursue this line of research as a career?

[Laughs]. It wasn’t what I intended to do. My original PhD research was in the oil industry, looking at reservoir cap rock. It’s really difficult to get good access to samples of reservoir cap rock, but at the end of the first year of my PhD I identified a location in the southeast corner of Taiwan. So I spent time there digging samples.

While I was there a typhoon formed and swept across Taiwan. I didn’t know anything about typhoons—except that they were windy and there was a danger from sea surge—so I thought it would be sensible to go up into the mountains to sit out the typhoon, which is what I did.

Actually, that’s not a good idea. While I was up in the mountains there was incredibly heavy rainfall from the typhoon and the slopes became very active.

When I finished my PhD and got my first academic position I didn’t have the equipment to undertake investigation of cap rock anymore, so I thought I would start looking at landslides. That’s what I’ve done ever since.

What conditions are in place when a landslide occurs? Is it typically a single failure or a combination of failures?

From the perspective of research there are two different sets of circumstances. One is when you have a trigger event that causes landslides across the landscape. I’ve done a lot of work on big earthquakes where there is seismic shaking over hundreds or even thousands of square kilometers and that triggers multiple landslides—sometimes a hundred thousand landslides or more in two minutes of shaking. We’ve seen that repeatedly in recent years.

Then there are also big rainstorms, like typhoons that bring extreme rainfall into mountain areas and trigger landslides across the landscape.

On the other hand, you sometimes get an individual failure event. Like the collapse of the side of a mountain. Or it might be something that humans trigger, via mining or road construction or whatever.

Aside from loss of life, what are some of the costs of landslides?

That’s a really good question; we don’t really know. You can get some isolated figures, most of which are not particularly well bench-marked. We know on average that landslides kill between five-thousand and fifteen-thousand people per year, and if there’s a really big earthquake, more than that.

But in terms of economic losses, it’s hard to calculate because landslides damage buildings but also infrastructure like railways and roads. They also destroy pipelines, and the economic losses from the destruction of a pipeline are very high. But we really have no proper sense as to what the number might be.

What measures can be taken to prevent landslides?

Except for the largest slides there are measures [you can take] to mitigate them. For most failures, the starting point if you want to try to prevent them is drainage. Most landslides are triggered by excess water.

But for over a hundred years now there have been techniques in place where you can basically put drains onto a slope. That draws down the water level , which makes the slope much less sensitive and reduces the chance of failure. If you want to prevent landslides, that’s the starting point.

And there are many other things that can be done. The most obvious is to dig the slope out and remove the loose material to stop it from failing. Or you can strengthen it by putting support onto the slope.
Part of my frustration about the high losses from landslides is that so many of them are preventable. It’s really a failure to invest or a failure to take proper engineering measures or a failure to recognize that there’s an instability problem.

Not long ago you wrote a post on how July 2019 was a record-breaking month for fatal landslides, which coincides with the warmest July on record. What does that tell us?

I’ve been following fatal landslides around the world since September 2002, so it’s a 17-year data set. In July 2019 we saw more fatal landslides than any previous month—driven by a surprisingly high number of landslides, not just in Asia, where we expect to see them, but in other parts of the world as well.

It’s not clear to me at the moment whether there is a link between it being the warmest July on record and the number of fatal landslides. The potential link is that we know that high atmospheric temperatures are associated with moisture in the air, and we know that as a general mechanism this is leading to an increase in rainfall intensity. So it could be that the summer rains that we typically see in July were associated with higher rainfall totals and/or higher rainfall intensities than in previous years, and that resulted in an increase in the number of landslides. That’s the hypothesis. But we have to do a lot more work to see whether that linkage is true.

But in general, we expect that as the planet becomes warmer and rainfall intensities increase, that we will see an increase in landslides. Slopes are very, very sensitive to higher rainfall intensities.

Alaska is one of the world’s hot spots in terms of earthquake frequency. Is there a place or country that consistently has more landslides than anywhere else?

Alaska is particularly interesting because there is good evidence that the number of really big mountainside collapses—the biggest landslides—have been increasing dramatically over the last decade, particularly in the southern parts of Alaska. Some really good research is being done looking at the number of these events and also where they are occurring. Part of that is being done by mapping [events] directly, but also by using earthquake sensors, which allow you to capture the larger landslide events.

There is clear, categorical evidence that the frequency of these big failures in Alaska is increasing quite rapidly with time. The timing is particularly interesting because the time of the year where this big increase is occurring is in the late spring and first part of the summer. The explanation for that is almost certainly that the higher atmospheric temperatures are causing the melting of the frozen ground—the so-called permafrost—in the mountains, which is creating these big landslides. So Alaska is a really good example of this coupling between wider environmental processes and slope failures.

But by far the biggest losses in terms of landslides occur in Asia, and in particular the Himalayas. The global hot spot in terms of landslide activity and human losses from landslides is in the arc that goes along the southern edge of the Himalayas—from Pakistan in the west across through India and Nepal and into Bangladesh. That arc of three-thousand kilometers or so accounts for about 75 percent of the landslides that we record. Most of those occur in the monsoon period—the period between June and October.

What is the world’s most famous landslide?

The most significant landslide—the one that everyone should know about—is the so-called Vajont Dam landslide, which occurred in northern Italy in 1963. It killed two-thousand or so people, and it was very, very large. But the reason it was important is that it was triggered by the construction of a dam and the impounding of a lake behind the dam, which caused a slope above the lake to become unstable. The slope was known to be moving, and believe it or not the operators of the dam were trying to stabilize the slope by increasing and lowering the level of the lake.

On October 9, 1963, the slope failed and this enormous landslide went into the lake and displaced the water out of the reservoir and over the top of the dam. The dam was about 250 meters high and the top of the wave was around 350 meters above that, so the wave that crashed down onto the bay below was over 500 meters—a third of a mile high—and it poured down onto a series of towns. Because it happened late at night there was very little warning for people in the valley below.

The Vajont Dam landslide triggered a huge amount of research into landslides and there have been many cases of landslides associated with dams and lakes since. But there has been no repeat of that sort of disaster, so it did result in dramatic improvements in our ability to monitor and manage slopes above reservoirs. I hope that as we build very large numbers of new dams in the high mountain areas of Asia that we are careful to remember the lessons of Vajont Dam.

The interesting thing is that the dam is still standing and the landslide is still sitting behind the dam, and if you go to the Dolomites in northern Italy you can go and visit the site. The dam looks enormous and makes you think about how the wave was that much higher.

At the other end of the scale, a really interesting case study is Hong Kong. In the 1970s, Hong Kong had a series of significant landslide accidents, mostly in very heavy rainfall. Most were collapses of slopes that swept down onto communities of refugees and people who had fled to Hong Kong, often from mainland China or Vietnam or other parts of Asia.

The problem became so serious that the government set up a program to manage slopes across Hong Kong, and that led to the formation of a government office that specialized in slope management—an office that still exists today.

It has been, by far, the most successful program in terms of slope management. Through a series of actions—one of which was to move people away from the most dangerous locations—they started to re-engineer dangerous slopes, both in the city and in the mountains. They also introduced standards for engineers and laws around slope maintenance.

So despite having had significant rainfall events they haven’t had a single fatality from landslides in Hong Kong for over ten years, which is the most beautiful illustration of where targeted action against this sort of hazard shows it’s possible to reduce the losses.

What is something that the general public doesn’t know about landslides that you think they should?

The first thing is that anywhere there are slopes there are landslides. One of the things we are trying to understand at the moment is the degree to which every slope might be moving. It’s probably reasonable to think that over a period of time pretty much every slope in existence is likely to go through some sort of failure. In many cases they are slow, very incremented and not significantly dangerous. Nevertheless, slopes do degrade with time.

A second thing is that landslides are extraordinarily varied phenomenon. I have seen landslide events that have led to loss of life where the size of the landslide was no bigger than a piece of stone you could hold in your hand. That is, a small rock fall had come off a cliff face and hit someone. At the other extreme, the biggest landslides on earth that aren’t submarine slides are half a mountain falling down.

But the vast majority of landslides are slow moving. The ones that move quickly are very much the exception. Ninety percent of moving slopes are slopes that are moving too slowly to see.

I did a lot of work in New Zealand around monitoring slopes and in one place we had a site where the town was built on what was very obviously a landslide. We put monitoring instruments in and we found that the thing was moving at one or two inches per year. It was creeping along but we could see how it varied, even seasonally.

At the other extreme, there is an avalanche type called a rock avalanche where a very large mountain rock mass collapses and falls vertically. Then it hits the bottom of a valley and the rock mass fragments, [becoming] a very fast moving avalanche of rocks. In some cases rock avalanches move at 200 or 300 kilometers per hour, and they can travel enormous distances—tens of miles.

How has smartphone video help advance the study of landslides?

It has been transformative. Around 15 years ago was the first time I saw a video of a landslide. And I remember thinking that I had no idea that’s that what it looked like, whereas now when you look on YouTube there are new videos several times a week. [Hence] the best landslide videos of 2019.